A technical feature of a screw thread structure is often adopted by a conventional fastening unit for engaging two plates, which generally comprises a screw, a cap and a sleeve. The screw comprises an end as a force receiving section provided with a screw groove, and another end comprising a screw rod connected with the force receiving section. The cap is fitted around a hollow cylinder about the force receiving section for encapsulating and protecting the screw. The sleeve is fitted around the screw rod. Accordingly, the above fastening unit is first engaged to a first plate by the sleeve, and the first and second plates are aligned with each other after moving the first plate, such that the screw rod of the screw corresponds to fastening openings of the first and second plates. Next, a rotational force is applied to the screw groove of the force receiving section of the screw with the aid of a tool, so as to fasten the screw rod into the fastening opening of the second plate and complete the assembly of the two plates. The above conventional screw thread structure is indeed capable of assembling two plates; however, it involves a more complicated assembling process since an external rotational force needs to be applied to the screw groove of the force receiving section of the screw with a tool for assembling.